The separation of NH3 from mixtures including NH3, acid gas containing H25 and/or CO2, and low-boiling water-soluble organic components, in particular waste water is known and based on the fact that NH3 forms azeotropes with the acid gases CO2 and H25 in aqueous solutions—The higher the temperature and the lower the NH3 concentration in the liquid, the poorer the azeotropes in acid gases. In a deacidifier, which operates at a high sump temperature, an acid gas free from NH3 thereby can be obtained as top product, whereas in the sump an aqueous solution is obtained, which contains the entire NH3 and only a small amount of the acid gases from the inlet. In an NH3 stripper, which operates at a low sump temperature and generally with a higher NH3 concentration, a bottom product is obtained, which contains the entire acid gases originating from the deacidifier sump. This requires a relatively small excess of NH3. The top product consists of NH3, which still contains a small concentration of acid gases. In an acid gas absorber, the acid gases are absorbed from the NH3 containing acid gas in aqueous NH3. In a total stripper, all volatile components are stripped from an aqueous solution of NH3, CO2, H25 and a number of further volatile and non-volatile components (depending on the origin of the feedstock), so that a waste water free from volatile components is obtained as bottom product.
Usually, the mixture containing NH3 is added to the sump of the acid gas absorber. To the top of the column, aqueous NH3 or pure water is charged, which absorbs NH3 in a cooled absorption circuit, whereby aqueous NH3 is formed, in which the acid gases are absorbed. The bottom product is supplied to the NH3 stripper, whose head vapors, which still contain some CO2, are introduced into the acid gas absorber. The bottom product of the NH3 stripper is supplied to the deacidifier. To the top of the deacidifier, pure water is added, which also can originate from the sump of the total stripper, in order to completely absorb NH3 from the acid gas. The bottom product of the deacidifier is supplied to the total stripper, whose top product is introduced into the sump of the acid gas absorber. The bottom product is NH3 and waste water free from CO2.
From DE 25 27 985 a variant of this process is known, in which the liquid feedstock is added to the deacidifier. One part of it is added preheated and at the lower part, whereas another part is added further above, in order to condense steam and absorb most of the NH3.
In the process disclosed in EP 0 212 690 B1, the main part of the liquid feedstock is added to the total stripper preheated, whereas the rest is supplied cold to the deacidifier. The top product of the total stripper is largely condensed and then added to the NH3 stripper further down than the sump of the acid gas absorber. This process is particularly favorable with a low acid gas concentration in the feedstock.
In the process according to EP 0 654 443 A1 deacidifier and total stripper are combined in one column. In the middle of the column, deacidified vapors are withdrawn and largely condensed at equal pressure. Further above, the non-condensed vapors are recirculated into the deacidifier. The liquid is introduced into the NH3 absorber, which is combined with the acid gas absorber to one column.
The process according to EP 1 135 331 B1 differs from the process described above in that the head vapors of the total stripper are condensed, wherein the liquid is introduced into the NH3 stripper.
In the process mentioned above, volatile water-soluble organic components in particular in the acid gas absorber can be enriched to such an extent that the processes do not work anymore.